You could turn on edge and vertex snapping for the polygon layer containing the void and use the Polygon tool to digitize a new feature inside the void.

You could use the Auto-Complete Polygon tool and draw a sketch across the void. This will create two new features in the void, which you could then merge.

You could copy and paste the feature, then delete the outer part of the new feature, leaving only the interior. This method works if the void is entirely contained within one polygon rather than at the border of two adjacent polygons.

If you have many voids to fill in with new features and want to do them all at once, you could set up a geodatabase topology with a Must Not Have Gaps rule. Using the Error Inspector or the Fix Topology Error tool , select all the errors caused by voids in the polygons, right-click, then click Create Feature.

Steps:

Click a polygon feature template in the Create Features window.

Click Trace on the Editor toolbar palette.

Right-click and click Trace Options or press the O key and make sure the offset is set to 0.

Click to start tracing, then click to stop after you have traced completely around the void.